The Effect of Soil Moisture on the Viscosity of Soil Paste

It is known from the literature that the rheological behavior of soils is largely dependent on the water content in pastes and soil organic matter forming the basis of organomineral soil gels. With an increase in soil moisture, gels can swell. As a result, the viscosity of the soil paste should change. The objective of this study was to assess the effect of soil moisture on the viscosity of soil paste. Arable soil horizons were used in this work: sod–podzolic, gray forest, leached chernozem, and chestnut. During the experiments, the soil moisture was changed, whereas the water content in the pastes in each soil type remained unchanged. The viscosity of the soil paste was determined by vibration viscometry, and the size of organomineral particles in pastes was determined by laser diffractometry. Two paste viscosity peaks depending on the soil moisture were obtained for all samples studied. The paste viscosity peaks were explained from the perspective of changes in the structure of humic substances in organomineral gels upon reaching critical concentrations: micelles–supramolecular formations–fractal clusters. Apparently, the transition between structural forms of humic substances under mechanical action on pastes is accompanied by the disintegration of large gel particles and the formation of a more balanced form of humic substances at a given water content.